1. Field of the Invention
The present invention relates to a shadow mask plate material and a shadow mask for use in a color-CRT.
2. Description of the Related Art
A shadow mask with a plurality of electron beam apertures is assembled into a color-CRT. The shadow mask has a function of projecting accurate electron beam spots onto a tricolor phosphor screen. For this reason, the relative positions, the aperture sizes, and the aperture shapes of the electron beam apertures have a direct influence on image quality, and so a high processing accuracy is required in formation of the electron beam apertures. In addition, to prevent occurrence of scattering electrons, it is also necessary to perform special processing for chamfering the edge of the electron beam aperture opposing the phosphor screen into a semispheric shape. If these processing accuracies are low, a decrease in image quality results due to doming. The electron beam apertures of a shadow mask as described above are formed by processing a shadow mask plate material by use of photoetching.
Recently, a general demand has increasingly arisen for a "high definition" of a TV screen, and the development of a high-definition TV system also has advanced in communication systems. Therefore, it is necessary to form finer electron beam apertures in a shadow mask for a color-CRT in order to improve its resolution.
To meet the above requirements, the use of a plate of an invar alloy such as a 3 wt % Ni-Fe alloy has been attempted. The invar alloy has a small thermal expansion coefficient. Therefore, a positional difference of electron beam apertures can be prevented in a shadow mask made from an invar alloy plate even if the temperature is raised due to bombardment of electron beams. Consequently, a color misregistration can be prevented. As an example, Jpn. Pat. Appln. KOKAI Publication No. 59-149638 discloses a shadow mask which has a recrystallized texture manufactured through steps of melting, hot forging, hot rolling, cold rolling intermediate annealing, adjustment rolling, and annealing for forming a recrystallized texture of an invar alloy as a raw material, and in which crystal faces on the surface are aligned in a {100} faces.
With increasing size and definition of a color-CRT, a shadow mask is also required to have more accurate, finer electron beam apertures. That is, in addition to having a small thermal expansion coefficient, a shadow mask plate material is required to allow easy and highly accurate formation of electron beam apertures which are fine and uniform in shape. However, when electron beam apertures are formed in the invar alloy-based plate material by photoetching, defective aperture shapes and white unevenness are found. This consequently make it difficult to improve image quality. More specifically, when desired electron beam apertures were formed by photoetching in the plate material disclosed in Jpn. Pat. Appln. KOKAI Publication No. 59-149638, in which the surface crystal faces were aligned in a {100} face, the electron beam apertures formed had an ideal similar figure. When observed microscopically, however, the sizes of these apertures varied from each other, and white unevenness caused by the difference in etched surface roughness was found.
Jpn. Pat. Appln. KOKAI Publication No. 4-341543, on the other hand, discloses an Fe-Ni-based shadow mask material which is manufactured by performing hot rolling, annealing, and cold rolling for an alloy containing 34 to 38 wt % of Ni and the balance consisting primarily of Fe, and in which the degree of aggregation of {111} crystal faces on the surface is 20% or more. This shadow mask material has a recrystallized texture and a high blackening processability resulting from the above definition of the degree of aggregation.